Aliphatic polyesters such as polyglycolic acid or polylactic acid are decomposed by water, microorganisms, or enzymes present in the natural world such as the ground or the sea and have therefore attracted attention as biodegradable polymer materials with a small environmental burden. In addition, since these aliphatic polyesters have biodegradable absorbent properties, they are also used as polymer materials for medical purposes such as surgical sutures or artificial skin. Among aliphatic polyesters, polyglycolic acid (hereafter called “PGA”) has a high melting point and can be melt-molded, and applications are therefore being developed for this substance alone or in conjugation with other resin materials or the like.
An aliphatic polyester can be synthesized by dehydrative polycondensation of α-hydroxycarboxylic acid with glycolic acid, lactic acid, or the like, for example, but it is difficult to produce a high-molecular-weight aliphatic polyester with this method. In contrast, in order to efficiently produce a high-molecular-weight aliphatic polyester, a method of synthesizing a dimeric cyclic ester of α-hydroxycarboxylic acid and subjecting the cyclic ester to ring-opening polymerization is employed. For example, when a glycolide, which is a dimeric cyclic ester of glycolic acid, is subjected to ring-opening polymerization, polyglycolic acid is obtained. When a lactide, which is a dimeric cyclic ester of lactic acid, is subjected to ring-opening polymerization, polylactic acid is obtained. An aliphatic polyester may also be obtained by the ring-opening polymerization of a lactone.
Known production methods for aliphatic polyesters using ring-opening polymerization of these cyclic esters include, for example, those described in Patent Documents 1 to 6 below. In addition, the present inventors have also proposed a production method for an aliphatic polyester using the ring-opening polymerization of a cyclic ester, wherein a partial polymer in a solid pulverized state is continuously obtained by continuously introducing a molten product of a partial polymer into a twin-screw stirring apparatus, further subjecting the product to solid phase polymerization, and then pelletizing the produced polymer by melting and kneading the polymer together with a thermal stabilizer (Patent Document 7).
In these production methods for aliphatic polyesters using the ring-opening polymerization of cyclic esters, an initiator (molecular weight modifier) such as an alcohol is used. A compound such as an oxide, a halogenide, a carboxylate, or an alkoxide of a metal such as tin (Sn), titanium (Ti), aluminum (Al), antimony (Sb), germanium (Ge), zirconium (Zr), or zinc (Zn), for example, is used as a ring-opening polymerization catalyst. Of these, tin compounds are preferably used since the catalytic activity of tin compounds is relatively higher than that of other metal compounds.
However, the improvement of the polymerization rate directly leads to the improvement of the productivity of the aliphatic polyester, so there are great expectations for further increases in the polymerization rate. One possible means for increasing the polymerization rate is an increase in the polymerization temperature, but increasing the polymerization rate by simply increasing the polymerization temperature induces a decrease in the equilibrium reaction rate based on the equalization of the polymerization rate and the depolymerization rate, which is not preferable. In addition, the problem of the discoloration of the produced polymer also occurs. One way to avoid such problems would be to improve the polymerization rate by using a co-catalyst. In addition, in the ring-opening polymerization using a metal compound catalyst, a tendency is discovered that the metal which remains in the produced polyester decreases the thermal stability of the polyester, promoting depolymerization and thermolysis of the polyester. Accordingly, it would be extremely desirable to find a co-catalyst that improves or does not decrease thermal stability in the ring-opening polymerization of a cyclic ester. However, practically no co-catalysts effective for the ring-opening polymerization of cyclic esters have yet been discovered.
For example, Patent Document 8 discloses the direct polymerization of lactic acid with a combination of a tin catalyst and an organic acid as a co-catalyst and describes that a polylactic acid with excellent thermal stability has been efficiently obtained as a result. However, it has been reported that the addition of an organic acid (octanoic acid) in ring-opening polymerization acts as a retardant in the production of a polylactic acid by ring-opening polymerization of a lactide (cyclic dimer of a lactic acid) using a tin compound catalyst (Non-Patent Document 1). On the other hand, in Non-Patent Document 2 and Patent Document 9, it is described that, in the ring-opening polymerization of a lactide or lactone using a tin compound catalyst, a Lewis base compound such as triphenylphosphine acts as a co-catalyst and leads to a reduction in polymerization time and an improvement in the thermal stability of the produced polyester. However, according to the research of the present inventors, the co-catalytic action of Lewis base compounds is not yet satisfactory.